When I first started Greek I asked my pastor (who is very efficient in reading Biblical Greek) if he could write a journal if he were given the proper supplemental vocabulary. He said that it probably wouldn't be possible. So, my plan has been that once I finished my year one course in Koine I would write him a letter in Koine explaining why it's possible and that he was wrong XP. Please critic this so I can show him a good example of living Koine.

I haven't done composition myself, but when I see others doing it, morphology or grammatical rules aren't usually a big problem. It's mostly about syntax and discourse, or how ideas are put into language idiomatically. Couple of details which don't somehow feel like good native Koine (I may be wrong):

σὺ μνημονεύεις: I would leave off the pronoun. Pronouns should be used sparingly and only when needed. Wooden English->Koine translations can often be detected by needless pronouns.

I wouldn't use double ὅτι. Rather replace some indicative with a participle, or something like that. Using participles in right places is impressive and in my opinion a mark of relatively good proficiency.

If you really want to impress, use hyperbaton. But it's even more difficult to compose fluently than participles, I guess.

Look at the opening of 2nd or 3rd John. There is an expression of feeling. In the letters recorded in Acts there is an expression of respect. How do you see your pastor emotionally or socially? Even though you are posting it here for criticism, it is a personal document. Personalise it.

"My pastor" sounds very modern to me. The function of that dative is to identify the recipient and to spell out the relationship between the parties, or to show what the writer thinks of the recipient's relationship to others. Could this letter be given to the right person at mail-call? Have you set a tone for the letter or hinted at your intent in writing? You could base a description of his pastor-ship in the language of New Testament. I'm in the middle of moving at present, so forgive me if I quote inaccurately, but the εις τον καταρτισμον των αγίων which accompanies the description of the role could be adapted with an appropriate verb for "appointed", δοθεντι or καθορισθεντι υπο Θεου. Changing των αγίων to ημων would be a way to imply the "my", rather than be so direct. Hiding in the crowd or putting your relationship in a bigger - social or cosmic - picture would be ways of avoiding self-agrandisement or hubris. Your social station in the Church and his relationship to God and in turn to you, your family and others sort of suggests than your self-description should be shorter or at least less ornate than what you say about him. You should use his name -think of the practicality of mail-call or delivery by an intermediary.